Coated Capillary Electrophoresis Tubes and System

ABSTRACT

The invention is directed to a capillary tube for electrophoresis that has a positively charged coating on the capillary inner surface that prevents positively charged analytes from adsorbing to the inner capillary surface. The capillary tube has an inner surface that is coated with a first polymer layer having a plurality of polymer groups comprising polyethylene imine, designated herein as (CH 2 CH 2 NH) X . The inner surface of the capillary typically has a second polymer layer covalently bonded to the first polymer layer. The invention includes a capillary tube where two or more than two polymer groups are covalently bonded to each other by a cross-linker. Also provided are an electrophoresis system the uses the coated capillary tubes, a method of performing electrophoresis that utilizes the coated capillary tubes, and a process for preparing the coated capillary tubes.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/150,650, filed on Jun. 10, 2005, which in turn is a continuation ofU.S. patent application Ser. No. 10/238,703, filed on Sep. 9, 2002, nowU.S. Pat. No. 6,923,895, issued Aug. 2, 2005, the entire contents of allof which are hereby incorporated herein by reference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

Not applicable

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK

Not applicable

BACKGROUND OF THE INVENTION

The following description provides a summary of information relevant tothe present invention and is not an indication that any of theinformation provided or publications referenced herein is prior art tothe presently claimed invention.

The electrokinetic separation of most analytes by electrophoresis istypically performed using buffers with wide range of pH (pH 3-10).Analytes such as peptides and proteins are positively charged underthese buffer conditions. A popular and efficient way of performingelectrokinetic separation on multiple samples is by capillary gelelectrophoresis. Capillary gel electrophoresis systems typically employfused silica capillary tubes. The surface of these fused silicacapillary tubes is negatively charged above pH 2.0 because the silanolgroups on the capillary surface become ionized. Consequently, thepositively charged proteins and peptides traveling through the capillaryduring electrophoresis interact with the surface of the fused silicacapillary and adsorb to the inner surface of the capillary tube. This isa problem because the adsorption of analytes continuously changes thesurface properties of the capillary tube, resulting in changes in theelectrosmotic flow velocity of the solution in the capillary. Thiscauses a fluctuation of migration time of analytes which results ininaccuracies and poor reproducibility.

Attempts have been made to mitigate the adsorption of positively chargedanalytes to negatively charged fused silica capillaries. For example,capillary electrophoresis has been performed using running buffers withpH 2-4 to reduce the ionization of surface silanol groups. This approachcan reduce the adsorption of analytes to a limited extent, but resultsin a reduction in sample throughput through the capillary andconsequently longer run times. Also, attempts have been made to coverthe capillary inner surface with neutral coatings. This approach alsoresults in a reduction in sample throughput through the capillary.Capillary tubes having a positive coating have also been made, wheremonomeric amine silanes are covalently bonded to the capillary surface.A problem with these coatings is that they are unstable under acidic andbasic conditions. None of the approaches used thus far have been ideal.What is needed are capillary tubes that are coated to obtain a longerrun life, that can typically be used one hundred or more analysis withreproducible results, and that can be prepared by a fast and simpleprocess. Also needed are capillary tubes and a capillary electrophoresissystem that is useful for high-throughput analysis of proteins,peptides, and other analytes that adhere to capillary tubes duringelectrophoresis.

BRIEF SUMMARY OF THE INVENTION

The invention satisfies this need. The invention provides a capillarytube for electrophoresis that has a positively charged coating on thecapillary inner surface that prevents positively charged analytes fromadsorbing to the inner capillary surface. The capillary tube comprisesan inner surface and an outer surface. At least a portion of the innersurface is coated with a first polymer layer having a plurality ofpolymer groups comprising (CH₂CH₂NH)_(x) attached by a first linkergroup to a first anchor group. The first anchor group is covalentlybonded to the inner surface of the capillary.

The inner surface of the capillary typically further comprises a secondpolymer layer covalently bonded to the first polymer layer. As comparedto a single layer, this additional layer results in a higher density ofpositive charges on the wall surface that facilitates an increase insample flow through the capillary. The second polymer layer has aplurality of polymer groups comprising (CH₂CH₂NH)_(x) attached by asecond linker group to a second anchor group that is covalently bondedto a first anchor group of a polymer group from the first polymer layer.In this formula, “x” represents an integer independently selected forindividual polymer groups within the first polymer layer or secondpolymer layer. The first polymer layer covalently bonded to theinner-surface of the capillary tube and the second polymer layercovalently bonded to the first polymer can be represented as illustratedbelow, where CS represents the inner surface of the capillary tube, N isnitrogen, O is oxygen, Si is silicon, x and y are integers between 7 and15 independently selected for individual polymer groups, R₁ is selectedfrom the group consisting of hydrogen, alkyl groups and halogens, R₂ andR₃ are independently selected from the group consisting of hydrogen,alkyl groups, and (CH₂CH₂NH)_(z) where z is an integer between 1 and 15,n and m are integers between 1 and 8 that are independently selected forindividual polymer groups, CR is a cross-linker that covalently bonds toone or more (CH₂CH₂NH) moiety of one polymer group to one or more(CH₂CH₂NH) moiety of another polymer group.

The invention includes a capillary tube for electrophoresis having aninner surface coated with a first polymer layer having the followingstructure:

Where CS is the inner surface of the capillary tube, N is nitrogen, O isoxygen, Si is silicon, R₁, R₂, R₃, and R₄ are independently selectedfrom the group consisting of hydrogen, alkyl groups and halogens, R₅ andR₆ are independently selected from the group consisting of hydrogen,alkyl groups, and one or more than one (CH₂CH₂NH) moiety, where y and yare integers between 7 and 15, n and m are integers independentlyselected for individual polymer groups that is between 1 and 8, CR is across-linker that covalently bonds to one or more (CH₂CH₂NH) moiety ofone polymer group to one or more (CH₂CH₂NH) moiety of another polymergroup.

Also provided is a capillary electrophoresis system comprising acapillary tube coated according to the description herein, means forsupporting the capillary tube, means for introducing a sample onto thecapillary tube, means for performing electrophoresis on the sample, andmeans for detecting the sample. In one embodiment, the systemincorporates a capillary tube having the following structure:

where CS is the inner surface of the capillary tube, N is nitrogen, O isoxygen, Si is silicon, x and y are integers independently selected forindividual polymer groups that are between 7 and 15, R₁ is selected fromthe group consisting of hydrogen, alkyl groups and halogens, R₂ and R₃are independently selected from, the group consisting of hydrogen, alkylgroups, and (CH₂CH₂NH), where z is an integer between 1 and 15, n and mare integers independently selected for individual polymer groups thatare between 1 and 8, CR is a cross-linker that covalently bonds to oneor more (CH₂CH₂NH) moiety of one polymer group to one or more (CH₂CH₂NH)moiety of another polymer group.

Further provided is a method of performing electrophoresis comprisingthe steps of providing a capillary electrophoresis apparatus, selectinga capillary tube according to the description herein, filing thecapillary tube with a gel to form a capillary gel, providing a samplecomprising one or more than one compound, loading the sample onto thecapillary gel, performing electrophoresis on the sample, and detectingone or more than one compound from the sample.

Finally, the invention includes a process for the preparation of asilica capillary tube with a coating comprising the steps of treatingthe inner surface of the silica capillary tube with a base, treating theinner surface of the capillary tube with a solution containing between2% and 30% trimethoxysilylpropyl (polyethyleneimine), rinsing the innersurface of the capillary tube, and treating the inner surface with across linker to cross link the polymer groups.

BRIEF DESCRIPTION OF THE DRAWINGS

These features, aspects and advantages of the present invention willbecome better understood with regard to the following description,appended claims and accompanying figures where:

FIG. 1 illustrates an electrophoresis separation profile produced fromsamples run on coated capillary tubes according to the inventionelectrophoresed in a separation buffer of 5% acetonitrile in 95% 0.5Nacetic acid at 400 v/cm field strength. Group 1A represents first fiveruns of five peptide standards. Group 1B represents 200 runs of horseheart cytochrome c digest.

FIG. 2 illustrates an electrophoresis separation profile produced fromsamples run on uncoated prior art capillary tubes in a separation bufferof 5% acetonitrile in 95% 0.5N acetic acid. Group 2A represents horseheart cytochrome c run at 400 v/cm. Group 2B represents horse heartcytochrome c run at 666 v/cm.

DETAILED DESCRIPTION OF THE INVENTION

The following discussion describes embodiments of the invention andseveral variations of these embodiments. This discussion should not beconstrued, however, as limiting the invention to those particularembodiments. Practitioners skilled in the art will recognize numerousother embodiments as well. In all of the embodiments described hereinthat are referred to as being preferred or particularly preferred, theseembodiments are not essential even though they may be preferred.

Chemical Structure of Polymer Groups

The invention is directed to coated capillary tubes for electrophoresis.The capillary tubes have an inner surface and an outer surface. Theinner surface of a capillary tube is coated with a first polymer layercomprising a plurality of polymer groups. Each polymer group comprises xrepresenting an integer that is typically between 1 and about 50 foreach polymer group. More typically, x is between 7 and 15 for eachpolymer group. In preferred embodiments, substantially all of thepolymer groups comprise linear chains of (CH₂CH₂NH)_(x) covalentlybonded to nitrogen. In other embodiments, the polymer groups comprisebranched chains where one or more than one nitrogen is a secondary ortertiary amine covalently bound to one or more (CH₂CH₂NH)_(x) groups.

One or more (CH₂CH₂NH)_(x) moiety of a polymer group is attached by afirst linker group to a first anchor group. Typically, the first andsecond linker group is an alkyl group having between 1 carbon atom and 6carbon atoms. In one embodiment, the alkyl group has 3 carbon atoms andthe alkyl group is a propyl group. Larger alkyl groups are possible, andchemical moieties other than alkyl groups can that covalently bond to(CH₂CH₂NH)_(x) moieties can be used as linkers. The first anchor groupis covalently bonded to the inner surface of the capillary. In theembodiment illustrated in structure I below, the anchor group comprisesa silicon atom bonded to constituent groups R₁ and R₂ selected from thegroup consisting of hydrogen, alkyl groups, alkoxy groups, and halogens,and the linker group is an alkyl group with the number of carbonsrepresented, by the integer n.

Typically the inner surface of the capillary tube comprises a silica,and the Si of anchor group is covalently bonded to the silica surface.Alternatively, the anchor group is another chemical moiety or a chemicalmoiety of the linker group that is covalently bound to the inner surfaceof the capillary. As illustrated in structure II below, embodimentswhere one polymer group is attached to a capillary surface through morethan one anchor group are within the scope of the invention.

Cross-Linking of Polymer Groups

In another aspect of the invention, one or more (CH₂CH₂NH)_(x) groupfrom two or more than two different polymer groups are covalently bondedto each other by a cross-linker. The capillary tube has an inner surfaceand an outer surface, with the inner surface being coated with a firstpolymer layer. The first polymer layer comprises a plurality of polymergroups, each comprising (CH₂CH₂NH)_(x), where x is an integer greaterthan 5. In alternative embodiments, x is an integer between 1 and 50,and more typically between 7 and 15. Two or more than two polymer groupsare covalently bonded to each other by a cross-linker. Typically, thecross-linker is selected from the group consisting of 1,4-butanedioldiglicidylether (BUDGE), ethylene glycol diglycidyl ether, triglycidylglycerol, and diglycidyl glycerol. Each polymer group is attached by alinker group to an anchor group, and the anchor group is covalentlybonded to the inner surface of the capillary.

In an exemplary embodiment, the inner surface of a capillary tube iscoated with a first polymer layer having the following structure:

where CS represents the inner surface of the capillary tube, N isnitrogen, 0 is oxygen, Si is silicon, R₁, R₂, R₃, and R₄ areindependently selected from the group consisting of hydrogen, alkylgroups and halogens, R₅ and R₆ are independently selected from the groupconsisting of hydrogen, alkyl groups, and one or more than one(CH₂CH₂NH) moiety, where y and y are integers between 7 and 15, n and mare integers independently selected for individual polymer groups thatis between 1 and 8, CR is a cross-linker that covalently bonds to one ormore (CH₂CH₂NH) moiety of one polymer group to one or more (CH₂CH₂NH)moiety of another polymer group. Preferably, R₁, R₂, R₃, and R₄ arehalogens. Preferably R₅ and R₆ comprise (CH₂CH₂NH)_(x), where x is aninteger greater between 1 and 6.

In a preferred embodiment, the cross-linker is 1,4-butanedioldiglicidylether and the coated capillary has structure (IV) illustratedbelow, where n is an integer between 1 and 7 and R representsconstituent groups independently selected from hydrogen, alkyl groups,alkoxy groups and halogens.

Additional Polymer Layers

Variations of this capillary tube further comprise a second polymerlayer covalently bound to the first polymer layer, and optionallyadditional polymer layers covalently bonded to successive layers. Anexemplary structure having a first polymer layer and a second polymerlayer is illustrated in structure V below, where each R constituentgroup is independently selected from the group consisting of hydrogen,alkyl groups, alkoxy groups and halogens, and n is an integer between 1and 7.

In one embodiment, one or more polymer group from the first polymerlayer and one or more polymer groups from the second polymer layer arecovalently bonded by a cross-linker. A capillary having a first and asecond polymer layer can, as an example, have structure (VI) illustratedbelow.

where CS is the inner surface of the capillary tube, N is nitrogen, O isoxygen, Si is silicon, x and y are integers between 7 and 15independently selected for individual polymer groups, R₁ is selectedfrom the group consisting of hydrogen, alky groups and halogens, R₂ andR₃ are independently selected from the group consisting of hydrogen,alkyl groups, and (CH₂CH₂NH)_(x) where z is an integer between 1 and 15,n and m are integers between 1 and 8 that are independently selected forindividual polymer groups, CR is a cross-linker that covalently bonds toone or more (CH₂CH₂NH) moiety of one polymer group to one or more(CH₂CH₂NH) moiety of another polymer group. More typically, the integersn and m are between 3 and 5. Alternatively, one or more polymer groupfrom the same polymer layer are cross-linked by a cross-linker, or morethan two polymer layers are present and cross-linked to other layers.

Electrophoresis System

The invention further includes a capillary electrophoresis systemcomprising a capillary tube as described herein, means for supportingthe capillary tube, means for introducing a sample onto the capillarytube, means for performing electrophoresis on the sample, and means fordetecting the sample. Exemplary capillary electrophoresis systems usefulfor performing electrophoresis include the P/ACE 2000 series, P/ACE 5000series or P/ACE MDQ Capillary Electrophoresis Systems (BeckmanInstruments, Inc., Fullerton, Calif.). Other capillary electrophoresissystems are described in “CEQ 2000 DNA Analysis System Performance”,Nora M. Galvin et al., Technical Information #T-1854A, Beckman Coulter,Inc., 1998; U.S. Pat. No. 5,120,413, by Chen; Fu-Tai A. et al., entitled“Analysis of samples utilizing capillary electrophoresis”; U.S. Pat. No.5,228,960, by Liu; Cheng-Ming et al., entitled “Analysis of samples bycapillary electrophoretic immunosubtraction”; U.S. Pat. No. 5,891,313 byJohnson; Ben F., et al., entitled “Entrapment of nucleic acid sequencingtemplate in sample mixtures by entangled polymer networks”, the contentsof which are all hereby incorporated by reference in their entirety.

The capillary tube of the electrophoresis system can be represented bystructure (VII) illustrated below.

In the above structure CS represents the inner surface of the capillarytube, N is nitrogen, O is oxygen, Si is silicon, x and y are integersindependently selected for individual polymer groups that are between 7and 15, R₁ is selected from the group consisting of alkyl groups andhalogens, R₂ and R₃ are independently selected from the group consistingof hydrogen, alkyl groups, and (CH₂CH₂NH)_(Z) where z is an integerbetween 1 and 15, n and m are integers independently selected forindividual polymer, groups that are between 1 and 8, CR is across-linker that covalently bonds to one or more (CH₂CH₂NH) moiety ofone polymer group to one or more (CH₂CH₂NH) moiety of another polymergroup.

Electrophoresis Methods

The invention further includes a method of performing electrophoresis.The method comprises the steps of providing a capillary electrophoresisapparatus, selecting a capillary tube describes herein, filing thecapillary tube with a gel or separation buffer to form a capillary gel,providing a sample comprising one or more than one compound; loading thesample onto the capillary gel, performing electrophoresis on the sample,and detecting one or more than one compound from the sample. FIGS. 1 and2 illustrate the enhanced electrophoretic separation that is achieved byusing the coated capillary tubes according to the invention. Theconditions for performing electrophoresis are described in more detailin Example II below.

Process for Preparing Coated Capillary Tubes

The invention further includes a process for the preparation of a silicacapillary tube with a coating. The process generally comprises the stepsof sequentially treating the inner surface of the silica capillary tubewith a base; acid, water and methanol; treating the inner surface of thecapillary tube with a solution containing between 2% and 30%trimethoxysilylpropyl (polyethyleneimine); rinsing the inner surface ofthe capillary tube; and treating the inner surface with a cross linkerto cross link the polymer groups. The detailed steps for preparing acoated capillary tube according to the invention are described below inExample 1.

Example I Preparation of Silica Capillary Tubes with a Cross-LinkedCoating

Trimethoxysilylpropyl (polyethyleneimine) (PEI silane) is a linearpolyethyleneimine attached to trimethoxy silyl propyl groups. PEI silanewas dissolved in methanol to obtain a 5% (v/v) solution of PEI silane inmethanol by vortexing for 3 min in a glass vial. A capillary tube wasfirst treated with methanol, then water, and then with 1.0 N SodiumHydroxide and 1.0 N hydrochloric acid. The capillary was rinsed withdeionized water, and afterward with methanol. The PEI silane solutionwas pumped through the capillary for 12-16 h at room temperature. Afterthis, the capillary was rinsed briefly with methanol. Next, a 10% (v/v)solution of 1,4-Butanediol diglicidylether (BUDGE) in 1,4-Dioxane waspumped through the capillary for 4-6 h at room temperature to cross-linkthose polymer chains. After this; nitrogen was passed through thecapillary while heating it for 1.0 h at 80° C. The capillary was thenrinsed thoroughly with methanol. The cross-linking of the polymer chainson the capillary surface enhances the stability of the coating. Thetrimethoxysilylpropyl (polyethyleneimine) forms a covalent bond betweenthe polymer and the silica capillary surface via trimethoxy silanegroups attached to the polymer. Amine groups present in the polymerelectrostatically bind to the surface by interacting with silanol groupson the surface. This creates a tightly bound layer of PEI silane on thesurface. Crosslinking of these polymer molecules is achieved by reactingamine groups of the polymer with the epoxy groups in BUDGE. Capillarytubes coated by this process provides a stable coating with a enhancedcoating coverage on the silica capillary surface, and in use thecapillaries effectively suppress the adsorption of analytes and maintaina high electrosmotic flow.

Example II Comparison of Electrophoresis Run Profiles for Coated andUncoated Capillary Tubes

FIG. 1 illustrates separation profiles obtained from PEI silane andBUDGE coated fused silica capillary tubes. The capillary tubes had a 40μm internal diameter, a 364 μm outer diameter, a 31 cm total length, andwere coated according to example I. Electrophoresis was performed at 400v/cm field strength on a Beckman Coulter P/ACE MDQ capillaryelectrophoresis system, available from Beckman Instruments, Inc.(Fullerton, Calif.). Group IA of FIG. 1 shows the electropherogramsobtained from the first five runs of five peptide standards. Group 1B inFIG. 1 shows electropherograms obtained up to 200 runs of Horse heartCytochrome c. The electropherogram shows that migration time of analyteswere stable from the first run and thereafter, and that theelectrophoretic separation took only 2.3 min to complete. FIG. 2illustrates separation profiles obtained from bare uncoated fused silicacapillary tubes having a 40 μm internal diameter, a 364 μm outerdiameter, and a 31 cm total length. In the experiments illustrated inFIGS. 1 and 2 the separation buffer used for electrophoresis was 5%acetonitrile in 95% 0.5N acetic acid. In FIG. 2, the bottom three groupsof electropherograms were obtained at 400 v/cm field strength while theother upper three groups of electropherograms were obtained at 666 v/cmfield strength. The bottom two groups show that the electrophoreticmigration of peptides was not stabilized at the beginning of the run, asit was when the coated capillary tubes of the invention were used, andthat it took about 7 min to complete the separation.

The experiments described in Example II demonstrate that the coatedcapillary tubes of the invention produced electrophoresis runs resultingin faster analyte separation and improved run-to-run reproducibility.

Having thus described the invention, it should be apparent that numerousmodifications and adaptations may be resorted to without departing fromthe scope and fair meaning of the instant invention as set forthhereinabove and as described hereinbelow by the claims.

Although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. Therefore, the spirit and scope of the appended claims shouldnot be limited to the description of the preferred versions describedherein.

All features disclosed in the specification, including the claims,abstracts, and drawings, and all the steps in any method or processdisclosed, may be combined in any combination, except combinations whereat least some of such features and/or steps are mutually exclusive. Eachfeature disclosed in the specification, including the claims, abstract,and drawings, can be replaced by alternative features serving the same,equivalent or similar purpose, unless expressly stated otherwise. Thus,unless expressly stated otherwise, each feature disclosed is one exampleonly of a generic series of equivalent or similar features:

Any element in a claim that does not explicitly state “means” forperforming a specified function or “step” for performing a specifiedfunction, should not be interpreted as a “means” or “step” clause asspecified in 35 U.S.C: §112.

What is claimed is:
 1. A process for the preparation of a surfacecomprising: a) treating the surface with a base; b) treating the surfacewith a solution containing between 2% and 30% trimethoxysilylpropyl(polyethyleneimine) to form a polymer group; c) rinsing the surface; andd) treating the surface with a cross-linker to cross-link at least onepolymer group.
 2. A process for the preparation of a silica capillarytube with a coating comprising the steps of: a) treating the innersurface of the silica capillary tube with a base; b) treating the innersurface of the capillary tube with a solution containing between 2% and30% trimethoxysilylpropyl (polyethyleneimine) to form a polymer group;c) rinsing the inner surface of the capillary tube; and d) treating theinner surface with a cross-linker to cross-link at least one polymergroup.